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Chapter 20 Mountain Building and the Evolution of Continents

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Orogenesis the processes that collectively produce a mountain ... Airy model of crustal root. 25. Erosion & Isostatic Adjustment. Young Mountains. Thick crust ... – PowerPoint PPT presentation

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Title: Chapter 20 Mountain Building and the Evolution of Continents


1
Chapter 20 Mountain Building and the Evolution
of Continents
GEOL 101 Introductory Geology
2
Mountain Belts
  • Orogenesis the processes that collectively
    produce a mountain belt
  • folding and thrust faulting
  • metamorphism and igneous activity
  • Recent mountain building
  • Alpine-Himalayan chain
  • American Cordillera
  • Island Arcs in western Pacific
  • Older mountain building
  • Appalachians, eastern US
  • Urals, Russia

3
Major Mountain Belts
4
Mountain Belts
  • Plate tectonics theory provides a good model for
    orogenesis
  • Mountain building at convergent boundaries
  • Aleutian-type
  • Andean-type
  • Continental collisions
  • Continental accretion
  • Mountain building away from plate margins

5
Convergent Boundaries
  • Aleutian-type mountain building
  • Where two ocean plates converge and one is
    subducted beneath the other
  • Located on margin of shrinking ocean basins
  • Most are found in the Pacific
  • Volcanic island arcs result from the steady
    subduction of oceanic lithosphere
  • forms on ocean floor
  • partial melting of mantle above subducted plate
  • Mountainous topography consisting of igneous and
    metamorphic rocks

6
Volcanic Island Arc
7
Aleutian Island Arc, Alaska
8
Convergent Boundaries
  • Andean-type mountain building
  • Mountain building along continental margins
  • Convergence of an oceanic plate and a plate whose
    leading edge contains continental crust
  • volcanic and tectonic features located inland of
    continental margin
  • Exemplified by the Andes Mt., South America
  • Stages of development
  • passive margin
  • active continental margins

9
Andean-Type Mountains
  • Passive margin
  • Continental margin is part of the same plate as
    the adjoining oceanic crust (not a plate
    boundary)
  • Sediment deposition on continental shelf produces
    a thick wedge of shallow-water sediments
  • Active continental margins
  • Subduction zone forms, deformation begins
  • Oceanic plate descends 100 km, partial melting
    of mantle above subducting slab generates magma
  • Continental volcanic arc develops
  • Accretionary wedge
  • Deformed sedimentary and metamorphic rocks
  • Scraps of ocean crust

10
Andean-Type Subduction
Passive margin
Active continental margin Subduction zone
Igneous activity and deformation
11
Andes Mountains
12
Andes Mountains
13
Andean-Type Mountains
  • Composed of roughly two parallel zones
  • Volcanic arc
  • Develops on the continental block
  • Consists of large intrusive bodies intermixed
    with high-temperature metamorphic rocks
  • Accretionary wedge
  • Seaward segment
  • Consists of folded, faulted, and metamorphosed
    sediments and volcanic debris

14
Andean-Type Mountains
  • Sierra Nevada and Coast Ranges, California and
    Nevada
  • One of the best examples of an inactive
    Andean-type orogenic belt
  • Subduction of the Pacific Basin under the western
    edge of the North American plate
  • Sierra Nevada batholith is a remnant of a portion
    of the continental volcanic arc
  • Franciscan Formation of California Coast Range
    chaotic mixture of sedimentary rocks represent
    the accretionary wedge

15
Sierra Nevada and Coast Ranges
16
Continental Collisions
  • Convergence of two lithospheric plates, both
    carrying continental crust
  • Himalayan Mountains young mountain range,
    collision of India with the Eurasian plate about
    45 million years (my) ago
  • Appalachian Mountains 250 to 300 my ago,
    collision of North America, Europe, and Africa
  • Orogenesis here is complex, includes
  • subduction and igneous activity
  • collision of continental blocks
  • folding and uplift of the crust

17
Continental Collisions
18
Continental Collisions
Valley and Ridge Province of the Appalachian
Mountains Folded and faulted sedimentary strata
formed during several mountain building events
19
Convergent Boundaries
  • Continental accretion
  • Small crustal fragments collide and merge with
    continental margins
  • Responsible for many of the mountainous regions
    rimming the Pacific
  • Accreted crustal blocks are called terranes

20
Accreted Terranes
Paleomagnetic and fossil data indicate terranes
originated south of present locations migrated
1000s km north One exception Sonoma Terrane
may have migrated 1000 km south (Skalbeck et
al. 1989)
21
Vertical Crustal Movements
  • Isostatic adjustment
  • Vertical motions and mantle convection
  • Possible mechanism for crustal subsidence

22
Vertical Crustal Movements
  • Isostatic adjustment
  • Less dense crust floats on top of the denser and
    deformable rocks of the mantle
  • Concept of floating crust in gravitational
    balance is called isostasy
  • Higher mountains have deeper roots

23
The Principle of Isostasy
Thicker block floats higher
24
The Principle of Isostasy
Airy model of crustal root
25
Erosion Isostatic Adjustment
Young Mountains Thick crust
Erosion lowers mountains, crust rises in response
Continued erosion and uplift, thinner crust
26
Vertical Crustal Movements
  • Vertical motions and mantle convection
  • Buoyancy of hot rising mantle material
  • accounts for broad upwarping in the overlying
    lithosphere
  • Uplift whole continents, Southern Africa
  • Downward crustal displacements
  • Regions once covered by ice during last Ice Age
  • Continental margins where sediments are
    deposited, such as the Mississippi River delta
  • Circular basins found in the interiors of some
    continents (Illinois and Michigan basins)

27
Vertical Crustal Movements
  • Possible mechanism for crustal subsidence
  • May be linked to subduction of oceanic
    lithosphere
  • Subducting, detached lithospheric plate
  • Creates downward flow in its wake
  • Tugs on the base of the overriding continent
  • Continent floats back into isostatic balance
  • More observational data is needed to test the
    hypothesis

28
Mountain Building Away From Plate Margins
  • Rocky Mountains
  • Colorado Plateau
  • Basin and Range province
  • Black Hills
  • Bighorns

29
Mountain of Western US
30
Colorado Rockies Maroon Bells, Aspen, CO
31
Mountain Building Away From Plate Margins
  • Crustal thickness suggests elevation difference
    between Great Plains and the Rockies must be the
    result of mantle flow
  • Hot mantle may have provided the buoyancy to
    raise the Rockies, Colorado Plateau, and Basin
    and Range province
  • Upwelling associated w/ Basin and Range started
    about 50 my age, active today
  • Alt. hypothesis addition of terranes to North
    America produced the uplift

32
Mountain Building Away From Plate Margins
33
The Origin and Evolution of Continental Crust
  • Lack of agreement among geologists as to the
    origin and evolution of continents
  • Early evolution of the continents model
  • One proposal is that continental crust formed
    early in Earths history
  • Total volume of continental crust has not changed
    appreciably since its origin

34
The Origin and Evolution of Continental Crust
  • Gradual evolution of continents model
  • Continents have grown larger through geologic
    time by the gradual accretion of material derived
    from the upper mantle
  • Earliest continental rocks came into existence at
    a few isolated island arcs
  • Evidence supporting the gradual evolution of the
    continents comes from research in regions of
    plate subduction, such as Japan and the western
    flanks of the Americas

35
The Origin and Evolution of Continental Crust
Multistage evolution process
36
Precambrian Mountain Belts
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